Many industrial processes require precise control of various process fluids. For example, in the pharmaceutical and semiconductor industries, mass flow controllers are used to precisely measure and control the amount of a process fluid that is introduced to a process chamber. The term fluid is used herein to describe any type of matter in any state that is capable of flow. It is to be understood that the term fluid applies to liquids, gases, and slurries comprising any combination of matter or substance to which controlled flow may be of interest.
Conventional mass flow controllers generally include four main portions: a flow meter, a control valve, a valve actuator, and a controller. The flow meter measures the mass flow rate of a fluid in a flow path and provides a signal indicative of that flow rate. The flow meter may include a mass flow sensor and a bypass. The mass flow sensor measures the mass flow rate of fluid in a sensor conduit that is fluidly coupled to the bypass. The mass flow rate of fluid in the sensor conduit is approximately proportional to the mass flow rate of fluid flowing in the bypass, with the sum of the two being the total flow rate through the flow path controlled by the mass flow controller. However, it should be appreciated that some mass flow controllers may not employ a bypass, as such, all of the fluid may flow through the sensor conduit.
In many mass flow controllers, a thermal mass flow sensor is used that includes a pair of resistors that are wound about the sensor conduit at spaced apart positions, each having a resistance that varies with temperature. As fluid flows through the sensor conduit, heat is carried from the upstream resistor toward the downstream resistor, with the temperature difference being proportional to the mass flow rate of the fluid flowing through the sensor conduit and the bypass.
A control valve is positioned in the main fluid flow path (typically downstream of the bypass and mass flow sensor) and can be controlled (e.g., opened or closed) to vary the mass flow rate of fluid flowing through the main fluid flow path and provided by the mass flow controller. The valve is typically controlled by a valve actuator, examples of which include solenoid actuators, piezoelectric actuators, stepper actuators, etc.
Control electronics control the position of the control valve based upon a set point indicative of the mass flow rate of fluid that is desired to be provided by the mass flow controller, and a flow signal from the mass flow sensor indicative of the actual mass flow rate of the fluid flowing in the sensor conduit. Traditional feedback control methods such as proportional control, integral control, proportional-integral (PI) control, derivative control, proportional-derivative (PD) control, integral-derivative (ID) control, and proportional-integral-derivative (PID) control are then used to control the flow of fluid in the mass flow controller. In each of the aforementioned feedback control methods, a control signal (e.g., a control valve drive signal) is generated based upon an error signal that is the difference between a set point signal indicative of the desired mass flow rate of the fluid and a feedback signal that is related to the actual mass flow rate sensed by the mass flow sensor.
Many conventional mass flow controllers are sensitive to component behavior that may be dependent upon any of a number of operating conditions including fluid species, flow rate, inlet and/or outlet pressure, temperature, etc. In addition, conventional mass flow controllers may exhibit certain non-uniformities particular to a combination of components used in the production of the mass flow controller which can result in inconsistent and undesirable performance of the mass flow controller.
To combat some of these problems, a mass flow controller may be tuned and/or calibrated during production. Production generally includes operating the mass flow controller on a test fluid under a set of operating conditions and tuning and/or calibrating the mass flow controller so that it exhibits satisfactory behavior.
As known to those skilled in the art, the tuning and/or calibration of a mass flow controller is an expensive, labor intensive procedure, often requiring one or more skilled operators and specialized equipment. For example, the mass flow sensor portion of the mass flow controller may be tuned by running known amounts of a known fluid through the sensor portion and adjusting certain filters or components to provide an appropriate response. A bypass may then be mounted to the sensor, and the bypass tuned with the known fluid to reflect an appropriate percentage of the fluid flowing in the main fluid flow path at various known flow rates. The mass flow sensor portion and bypass may then be mated to the control valve and control electronics portions and then tuned again, under known conditions.
When the type of fluid used by an end-user differs from that used in tuning and/or calibration, or when the operating conditions, such as inlet and outlet pressure, temperature, range of flow rates, etc., used by the end-user differ from that used in tuning and/or calibration, the operation of the mass flow controller can be expected to degrade. For this reason, additional fluids (termed “surrogate fluids”) and or operating conditions are often tuned or calibrated, with any changes necessary to provide a satisfactory response being stored in a lookup table.
Although the use of additional tuning and/or calibration with different fluids and at different operating conditions can be used to improve the performance of the mass flow controller, this type of surrogate tuning and/or calibration is time consuming and expensive, as the tuning and/or calibration procedures must be repeated for at least each surrogate fluid and likely must be repeated for a number of different operating conditions with each surrogate fluid. Furthermore, because the surrogate fluids only approximate the behavior of the various types of fluids that may be used by the end-user, the actual operation of the mass flow controller at an end-user site may differ substantially from that during tuning and/or calibration. Considering the wide range of industries and applications employing mass flow controllers, the process fluid and operating conditions applied to the mass flow controller by an end user are likely to be different than the test fluids and operating conditions upon which a mass flow controller was tuned and/or calibrated, despite tuning and/or calibration of the mass flow controller with a number of different surrogate fluids and operating conditions.
In addition to the foregoing external factors (e.g., fluid species, flow rate, inlet and/or outlet pressure, temperature, etc.) that may affect the performance and response of a mass flow controller, factors associated with the physical operation of a mass flow controller may also contribute to the overall sensitivity of the mass flow controller to external factors and changing conditions. For example, many valves employed to control flow in mass flow controllers are solenoid actuated devices.
Although a number of manufacturers of mass flow controllers utilize piezoelectric actuators, solenoid actuators are generally preferred due to their simplicity, their quick response, and their low cost. Nonetheless, solenoid actuated control valves do have certain drawbacks, with one of the more significant drawbacks of solenoid actuated control valves (and solenoid actuated devices in general) being that they exhibit hysteresis. Hysteresis is a well known phenomenon common to many apparatus employing magnetics or electromagnetics or magnetic material. In general, hysteresis applies to a lagging or retardation in the values of resulting magnetization due to a changing magnetizing force. In many solenoid actuated devices, this results in a condition wherein the operation of the device depends not only upon a present state of the device, but also upon a prior state.
It is commonly understood that solenoid actuated control valves exhibit hysteresis. It is also commonly understood that this hysteresis adversely impacts the consistency of a valve with respect to transitioning between states of no flow and controlled flow in a mass flow controller. Nonetheless, in conventional mass flow controller design, this drawback has typically been accepted as a necessary drawback of using a solenoid actuated control valve, which, for many manufacturers, is outweighed by the advantages of solenoid actuated control valve, such as simplicity, cost, and reliability.